In my opinion, due to the physical appearance of pmp4000 and pmp3000
look the same, it will most likely also have a common circuit and parts list (at least, most).
Usually Behringer (and all the manufacturers), with the same series / sequence, using the same circuit, the difference is the number of channels only. You can compare the parts list on PMP3000 schematic, with your PMP4000.
PMP4000.
PMP3000
You can download from this link.
http://elektrotanya.com/behringer_pmp3000_sch.rar/download.html
Hope this helps.
Regards.